The present invention relates to a method for location of a cable in a dc connection in a bipolar 12-pulse system for transmission of electrical energy by means of high-voltage direct current, whereby the cable is located by sensing a magnetic field associated with a current flowing through the cable, and a device for carrying out the method.
A system for transmission of electrical energy by means of high-voltage direct current (HVDC) comprises two line-commutated converters, each one via a separate converter transformer connected to a separate three-phase ac network. Each one of the converters comprises two 6-pulse valve bridges, mutually series-connected at a connection point, each valve bridge being connected to a respective ac network via a respective secondary winding on the converter transformer. This is adapted such that the voltages from the respective secondary winding exhibits a mutual phase shift of 30 electrical degrees such that the two valve bridges of the converter in a known manner form a 12-pulse unit.
Via their dc voltage terminals, the converters are connected to each other by means of a dc connection with two dc conductors such that the transmission system will form a bipolar 12-pulse system.
For a general description of HVDC systems, reference is made to {dot over (A)}ke Ekstrxc3x6m: High Power Electronics HVDC and SVC. Stockholm 1990, inparticular pages 3-10 to 3-11 with FIGS. 3-11, and pages 7-1 to 7-4 and pages 7-8 to 7-13 with FIGS. 7-2.
The dc conductors may, at least in part, be made in the form of cables. When the energy is to be transmitted over a region with water, most often a sea region, the cables are usually buried beneath the sea bed to protect them from, for example, trawls from fishing boats and from anchors lowered from ships.
For location of the position of the cables during the burial stage, an electric current is fed through the cable where the position of the cable is determined in a known manner by means of magnetic sensors which sense the magnetic field generated by the current. Since, particularly in shallower waters, the sea bed may be subjected to movements, for example because of storms, it is also necessary, after a certain time of operation of the energy transmission, to check, at recurrent intervals, the position and depth of trench of the cables below the sea bed.
In those cases where the cables are laid close to each other on the sea bed or are arranged in the form of a common double cable, however, a circulating current in the dc connection, fed in via the converters, does not give rise to a reliably detectable external magnetic field. To carry out a check of the position of the cable, that part of the dc connection which consists of cable may, for example, be disconnected from the other parts of the dc connection and a current be supplied to either of the cables by means of an external current source. However, this implies relatively extensive work and during the time needed for this work and for restoration of the dc connection, the transmission system is not available for transmission of electrical energy.
The object of the invention is to achieve an improved method of the kind described in the introduction, which permits a location of the position of the cable while the transmission system is in normal operation, whereby the above-mentioned disadvantages concerning the availability of the transmission system for transmission of electrical energy can be avoided, and a device for carrying out the method.
According to the invention, this object is achieved by temporarily supplying to the dc connection a ground-mode current intended for the location, and by sensing a magnetic field associated with this ground-mode current.
According to an advantageous improvement of the invention, one of the connection points is connected to ground via an impedance circuit with an impedance, capable of being influenced, of a substantially inductive nature, whereby the ground-mode current is supplied to the dc connection by the inductance of the impedance circuit being temporarily reduced.
According to another advantageous improvement of the invention, the device comprises a current source connected between one of the connection points and ground.
According to still another advantageous improvement of the invention, the ground-mode current is supplied to the dc connection by bringing at least one of the converters to generate a ground-mode current.
Further improvements of the invention will become clear from the following description and claims.